Ink jet recording is an image recording technique comprising ejecting ink droplets from fine nozzle heads to form letters or figures on the surface of a recording medium such as paper. Ink jet recording technologies that have been practically used include a method of recording letters or figures on the surface of a recording medium wherein electrical signals are converted to mechanical signals by using a piezoelectric actuator to discontinuously eject ink droplets stored in the nozzle head and a method of recording letters or figures on the surface of a recording medium wherein part of ink is rapidly heated to generate a bubble near the ejecting part of a nozzle head to discontinuously eject ink droplets by the volume expansion of the bubble.
Aqueous pigment inks having a pigment dispersed in water have recently been provided as ink jet recording inks. This is because inks using pigments have characteristics of excellent water resistance and light resistance as compared with inks using water-soluble dyes. It is generally practiced that such aqueous pigment ink is prepared by dispersing a pigment in an aqueous medium with the aid of a dispersant such as a surface active agent or a polymeric dispersant.
For example, in Patent Document 1, in a pigment ink using an acetylene glycol-based penetrant, it is investigated to secure the dispersion stability of the ink by using a polymeric dispersant as a dispersant for pigment particles and using water, a non-volatile organic solvent, or a lower alcohol as an aqueous medium. However, use of a dispersant for dispersing pigment particles leads to an increased number of elements involved in ink preparation, which makes it difficult to control ink physical properties such as viscosity within desired ranges. Besides this, the pigment ink still has the problem that a sufficient print density is hard to obtain.
Furthermore, in the aqueous pigment ink, a dispersant is merely adsorbed on the surface of pigment particles. Since a strong shear force is applied to the ink fluid when it is ejected through a fine nozzle of a nozzle head, the dispersant may come off the surface of pigment particles to deteriorate its dispersing capability, which may result in a tendency of instable ink ejection. A tendency of instable dispersibility may also be observed when the aqueous pigment ink is stored for a long period of time.
A technology of introducing a sulfonic acid group on the surface of pigment particles has been proposed as a technique for dispersing the pigment particles in water. For example, Patent Document 2 discloses a pigment ink containing a surface-sulfonated organic pigment which is obtained by treating an organic pigment dispersed in a solvent having no active protons with a sulfonating agent (conventional art 1). According to the conventional art 1, the pigment ink has excellent dispersion stability and exhibits satisfactory ejection stability from the nozzle of a recording head (the property of being stably ejected from the recording head in a fixed direction).
Moreover, Patent Document 3 describes that an organic pigment mass whose surface is positively charged is prepared by treating a sulfonated organic pigment mass with a monovalent metallic ion and further describes an aqueous ink composition containing a pigment prepared from the positively surface-charged organic pigment mass, a dispersant, and water and having a high storage stability (dispersion stability) (conventional art 2).
Although the ink using the surface-treated pigment particles of the above conventional arts 1 or 2 as a colorant is excellent in dispersion stability and ejection stability as compared with conventional pigment-based ink jet recording inks, the images obtained by printing it on recording media such as plain paper or inkjet recording media (recording media having an ink receiving layer designed for receiving an ink jet recording ink) have still insufficient abrasion resistance. This seems to be due to insufficient fixability of the surface-treated pigment particles on the recording media.
On the other hand, a technology of employing a microencapsulated pigment wherein colorant particles are encapsulated with a polymer is known for the purpose of improving fixability of pigments containing in a pigment-based ink jet ink on a recording medium.
Patent Documents 4 and 5 propose encapsulated pigment particles and Patent Documents 6 to 9 propose pigment particles whose surface is grafted with a polymer. Patent Document 10 proposes a method of microencapsulating a hydrophobic powder with an amphiphilic graft polymer, which has turned out to involve the problem that use of the previously polymerized polymer results in too large particle diameter after encapsulation.
In addition to the above proposals, Patent Documents 11 to 19 disclose inks using pigments coated with a resin capable of forming film at room temperature by phase inversion emulsification. Patent Documents 20 to 29 propose inks using pigments coated with an anionic group-containing organic polymer by acid precipitation.
Furthermore, Patent Documents 30 to 35 propose inks using a polymer emulsion having polymer fine particles impregnated with a colorant by phase inversion emulsification (conventional art 3). However, even using the colorant obtained by phase inversion emulsification or acid precipitation, the polymer adsorbed on the pigment particles may sometimes separate and dissolve in the ink, resulting in insufficient performance in terms of dispersion stability, ejection stability, image quality, and the like of the ink, depending on the kind of organic solvents such as a penetrant to be used in the ink. Because the ink according to the conventional art 3 not a little suffers from separation of the adsorbed polymer from pigment particles, the pigment content in the ink is limited in view of dispersion stability. As a result, the ink only provides recorded matters with low print densities and, when plain paper is used as a recording medium, there arise problems that blurring of an image tends to occur and color developability is also low.
Patent Document 1: JP 03-157464 A
Patent Document 2: JP 10-110129 A
Patent Document 3: JP 11-409974 A
Patent Document 4: JP 07-094634 B
Patent Document 5: JP 08-059715 A
Patent Document 6: JP 05-339516 A
Patent Document 7: JP 08-302227 A
Patent Document 8: JP 08-302228 A
Patent Document 9: JP 08-081647 A
Patent Document 10: JP 05-320276 A
Patent Document 11: JP 08-218015 A
Patent Document 12: JP 08-295837 A
Patent Document 13: JP 09-003376 A
Patent Document 14: JP 08-183920 A
Patent Document 15: JP 10-046075 A
Patent Document 16: JP 10-292143 A
Patent Document 17: JP 11-080633 A
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Patent Document 19: JP 2000-007961 A
Patent Document 20: JP 09-031360 A
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Patent Document 23: JP 09-104834 A
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